The shapes of automobiles and metal components have become complex, and demand thereof has increased. Thus, besides traditional manufacturing methods such as forging and casting methods, methods optimized for mass production such as hot press forming (HPF) have been increasingly used. Owing to the development of HPF technology, the rigidity and other properties of products formed of metal powder have improved, and thus the use of HPF for manufacturing complex automobile components has been gradually increased. Therefore, atomization techniques for producing metal powder in large quantities have been researched. FIG. 1 illustrates a powder manufacturing apparatus for producing fine powder (P) by atomizing molten steel (S) using a fluid such as high-pressure gas or cooling water. The powder manufacturing apparatus may be used to produce micro-size fine powder having an intended particle size distribution and properties. Molten steel (S) flowing downward from a molten steel supply unit 10 is atomized into fine powder (P) by a fluid ejected onto the molten steel (S) from jet nozzles 30 mounted on a main body 20. The jet nozzles 30 are connected to a fixed body 11, and ejection positions of the jet nozzles 30 connected to the fixed body 11 are adjustable to vary a striking point at which a fluid ejected from the jet nozzles 30 strike molten steel (S).
A method of using inert gas as a fluid has merits such as the formation of very fine powder, uniformity in particle size, and nonoccurrence of powder oxidation, but has demerits in terms of mass production. On the other hand, although a water jet method using cooling water has demerits such as uneven particle surface shapes, difficulty in obtaining uniform particles, and a high possibility of metal powder oxidation, the water jet method has merits in terms of mass production. Since there is markedly increasing demand for metal powder as a raw material for manufacturing automobile components, the water jet method using cooling water is considered a competitive method for producing metal powder.
When metal powder is produced by the water jet method, the metal powder quality is determined by factors such as particle size distribution, apparent density, surface shape, and oxygen content of the metal powder. The particle size distribution, apparent density, and surface shape of metal powder are mostly determined in a water jet process, and variables of the water jet process such as the amount and pressure of cooling water, the initial temperature of molten steel, and the structures of nozzles have an effect on the properties of metal powder. In a general water jet process, molten steel is atomized into fine metal powder and cooled as high-pressure cooling water strikes the molten steel, and the atomization degree and the surface shape of the metal powder are determined by the pressure of the cooling water, specifically, the size and velocity of cooling water droplets and the magnitude of impulse applied by the cooling water droplets. Water jet nozzles and nozzle structures for forming water droplets and effectively atomizing molten steel by striking the molten steel with the water droplets have been developed and commercialized.
In the related art, such nozzle structures are generally classified into two types.
First, as illustrated in FIG. 2, a V-jet type nozzle structure is used. In the V-jet type nozzle structure, nozzle tips 31 are configured to eject fan-shaped streams of cooling water toward a point of a stream of molten steel so as to produce metal powder. The V-jet type nozzle structure includes a plurality of nozzle tips 31, and cooling water ejected through the nozzle tips 31 spreads widely. Thus, it is easy to set process conditions and adjust the angle at which cooling water strikes molten steel. However, the number of cooling water droplets effectively striking molten steel is relatively small, and thus a large amount of cooling water is used to produce powder.
As illustrated in FIG. 3, the other is a ring type nozzle structure including a ring-shaped one-piece nozzle 35 and ejection holes 36 through which streams of cooling water are ejected toward a point of molten steel. Compared to the V-jet type nozzle structure, a relatively great impulse is applied to molten steel by cooling water droplets (fluid droplets), and thus a less amount of fluid is used. However, if initial process conditions are not perfect, it is difficult to adjust the angle of fluid droplets with respect to a point of molten steel. In addition, it is difficult to manufacture the ring type nozzle structure in one-piece for high-pressure fluid ejection.
Moreover, in both the nozzle structures, if the striking angle at which a fluid strikes molten steel is varied, fine powder formed from the molten steel may not fall but may form large lumps depending on the flow of cooling water and air.
(Patent Document 1) KR10-2004-0067608 A